1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal device in which blueness is suppressed in black display, to thereby improve a contrast of display.
2. Description of the Related Art
The liquid crystal display device has such characteristics as having a high display quality, and being thin and low power consumption. Owing to those advantages, its applications are widened to various fields. In recent years, along with enlargement of its applications as monitors for mobile devices such as a mobile phone and a digital still camera, monitors for a desktop personal computer, monitors for printing and designing, monitors for a medical use, and further for liquid crystal televisions, there are increasing demands for excellent color reproducibility and a high contrast ratio. In particular, in the liquid crystal televisions, greater importance is placed on expression of a black color, and high luminance is also strongly required therefor.
A preference for color tone greatly influences on the image quality of the liquid crystal television. For example, in Japan, white display in the liquid crystal television is not set as an achromatic color in chromatics, but may sometime be set to 9,300 K, or further to 10,000 K or more, which being a high color temperature.
In a liquid crystal display device, which uses a pair of the polarizing plates for display, the transmission properties of a perpendicular polarizing plate and a parallel polarizing plate to be used as the polarizing plate are dominant for the white display and the black display. Specifically, a black color is affected by the perpendicular transmittance of the polarizing plate, and a white color is affected by the property of the parallel transmittance. A low perpendicular transmittance and a high parallel transmittance are required for attaining a high contrast ratio. As this type of polarizing plate, there is widely used a polarizing plate obtained by stretching an iodated base such as a polyvinyl alcohol resin. However, in a case of the polarizing plate with oriented iodine in a stretched resin, the contrast ratio in a shortwave region becomes lower in many cases. This reason is considered that it is difficult to control the order parameters of the resin and iodine completely.
Owing to this, the transmittance of the light corresponding to the shortwave region, i.e., blue region is higher in the black display and lower in the white display as compared with the transmitted light in the longwave region. When the white display is set so as to attain a high color temperature, namely, when the white display is set at a highly bluish white color, the blueness is intensified in the black display, resulting in lowering color tone reproducibility of the image in the liquid crystal televisions, in which the expression of a black color is considered important. As means for solving a color tone difference between black color and white color caused by the polarizing plate described above, a technology relating to a color tone correction polarizing plate is reported in “Silverstein, Louis D, and Paukshto, Michael V., Thin Crystal Films (TCF) for LCD Color Correction, SID 03 DIGEST (2003), pp. 824-827,” and a technology for compensating a wavelength dependency of the degree of polarization shown by the polarizing plate is reported in Japanese Patent Application Laid-open No. 2006-91393. Also, Japanese Patent Application Laid-open No. 2003-29724 can be cited as a technology relating to compensating color tone in dark region on a liquid crystal display of PVA (patterned vertical alignment) mode.
Description is made of a structural example of the liquid crystal display device with reference to FIG. 1. FIG. 1 is a schematic cross-sectional view of the liquid crystal display device. Principle of the liquid crystal display device resides in that linearly polarized light which is transmitted through an input side polarizing plate 118 is caused to change its orientation direction by a liquid crystal layer, thereby changing its polarizing state, and a light quantity transmitting through an output side polarizing plate 119 is controlled to achieve display. It is ideal that there is no change in its polarizing state by the liquid crystal layer in black display, and that the light from the light source is blocked with the output side polarizing plate 119 arranged at a right angle. Accordingly, the black display is a product of an orthogonal spectral transmittance of the polarizing plate to be used and a color filter spectral transmittance. That is, the polarizing plate and the color filter are almost dominant for the black display in spite of absorption due to a substrate, an insulating layer, a transparent electrode, or the like.
The half tones and white display obtained through the transmission of light are displayed due to the transmittance of birefringent light, which is generated by the liquid crystal layer, through the output side polarizing plate 119. Therefore, in the white display, a parallel spectral transmittance of the polarizing plate to be used, the birefringent light of the liquid crystal, and a spectral transmittance of the color filter almost dominantly act thereon. However, a polarizing degree of the polarizing plate with oriented iodine in a stretched resin such as polyvinyl alcohol is lowered in a short wavelength range, and hence in the black display, the black is colored into blue, and in the white display, the transmittance of blue is lowered. On the other hand, in the black display, leakage light may generate due to light scattering or the like caused by pigment particles forming a color filter layer, or the liquid crystal layer. As a result, the luminance is increased from the luminance of the ideal black display, which causes a change in color tone of the black display.
As described above, as to the display characteristics of a liquid crystal display device using polarizing plates, the color tones of a black display and a white display are largely varied mainly by the difference in spectral characteristics between the perpendicular transmittance and parallel transmittance of the polarizing plates. In particular, the black display is remarkably influenced by the orthogonal spectral characteristics of the polarizing plates. The iodine stretching type polarizing plates commonly used at present are disadvantageous as being not possible to block the orthogonal light at about 400 nm completely, resulting in a problem of expressing bluish black color.
JP 2003-29724 A discloses a technology for correcting the color tone through the control of three pixels of RGB independently. However, in order to achromatize the blue transmitted light, the transmission amounts of the green light and the red light are required to increase. When employing this method in the black display, resulting in increasing the luminance of the black display, and hence the contrast ratio is unavoidably decreased. In the liquid crystal television, in which the expression of the black color is considered important, the inductions of the luminance increase of the black display and the contrast ratio decrease are not allowable. In addition, displaying a black color at different orientation states of crystal liquid molecules in the pixels, respectively, of RGB becomes a cause for the deterioration of viewing angle characteristics. Therefore, the above-mentioned technology is not preferred also in this point.
JP 2006-91393 A discloses a technology in which a uniaxial absorptive anisotropic layer is provided between a pair of the polarizing plates of the liquid crystal display device, to thereby compensate wavelength dependency of the degree of polarization of the polarizing plate. Owing to intensive studies of the inventors of the present invention, it is revealed that, in order to attain effects such as reduction of blueness at the time of black display and improvement of the contrast ratio of the liquid crystal display device, the absorbance and a value of a dichroic ratio of the absorption axis and a light transmission axis of the uniaxial absorptive anisotropic layer are very important. For example, if the absorbance of the transmission axis of the uniaxial absorptive anisotropic layer is high, resulting in a cause of shift of the color tone of the white display. Besides, unless the absorbance of the absorption axis of the uniaxial absorptive anisotropic layer is a given value or more, a sufficient effect may not be obtained.
In the color tone correction polarizing plate, published in the above-mentioned Silverstein et al., for achromatizing perpendicular transmittance characteristics of the polarizing plates through the arrangement of the dye showing dichromatism in the shortwave range outside the respective pair of polarizing plates, four polarizing layers are formed, and hence a process for adjusting the axes of the individual layers becomes necessary. As a result, the load on the production process is inevitably increased. Further, the fluctuation of the degree of polarization of the polarizing plates causes a nonuniform display quality, which being a problem of productivity.